Variable closing resistor

ABSTRACT

A source of high-voltage electrical power and an electrical system, comprising transmission lines, which is connectable to the source. Switching means are provided to effect such connection. Impedance means, comprising closing resistors, are provided for insertion in series between the source and system upon closing of the switching means to reduce voltage surges in the system. Each closing resistor has a variable resistance value, i.e., a predetermined initial value when first inserted which is decreased at a predetermined rate within a predetermined time after insertion to effect improved reduction of voltage surges.

United States Patent [72] inventors Alfred E. Kilgour Brookfieid; James11. Harlow, Hales Corners, both of Wis. [21] Appl. No. 721,666 [22]Filed Apr. 16, 1968 [45] Patented Nov. 2, 1971 [73] AssigneeAllis-Chalmers Manufacturing Company Milwaukee, Wis.

[54] VARIABLE CLOSING RESISTOR 9 Claims, 4 Drawing Figs.

[52] U.S. Ci 307/93, 317/20, 200/148 [51] 1102b 3/22 [50] 307/93, 136,98, 92; 317/20, 16; 318/228; 200/148 [56] References Cited UNITED STATESPATENTS 3,291,947 12/1966 Van Sickle ZOO/148.8

614,394 11/1898 Kahlenberg 318/228 730,108 6/1903 Girault 3l8/228X916,343 3 1909 Lum 307/98 1,113,289 10/1914 Barnum... 3l8/228X 1,649,39111/1927 Davis 317/20x 2,639,357 5/1953 Kesselring... 307/93x 2,963,63712/1960 Osborn 307/93x 3,193,711 7/1965 Western 307/93x PrimaryExaminer-Herman J. Hohauser Attorneys-Thomas F. Kirby, Robert B. Bensonand Lee H.

Kaiser ABSTRACT: A source of high-voltage electrical power and anelectrical system, comprising transmission lines, which is connectableto the source. Switching means are provided to effect such connection.Impedance means, comprising closing resistors, are provided forinsertion in series between the source and system upon closing of theswitching means to reduce voltage surges in the system. Each closingresistor has a variable resistance value, i.e., a predetermined initialvalue when first inserted which is decreased at a predetermined ratewithin a predetermined time after insertion to effect improved reductionof voltage surges.

1 VARIABLE CLOSING RESISTOR SUMMARY OF THE INVENTION This inventionrelates generally to closing resistors which are used when connectingelectrical systems to a power source to limit voltage surges in thesystem. More particularly, the inv vention relates to variable closingresistors which are decreased in ohmic value after insertion in seriescircuit between the source and system.

Closing resistors (so called low ohmic value" resistors) are usedinhigh-voltage electrical transmission systems to limit cuit betweensource and system as the switch is closed and is subsequently steppedout when the switch is fully closed. Closing resistors could be used inany electrical system of any voltage level but are most useful in highvoltage or extra high voltage transmission systems on the order of 230kv. and having line lengths on the order of many miles. In the absenceof closing resistors, surge voltages may be as high as five-times-normalsystem peak voltage and extra costly insulation is therefore required.Heretofore, it was the practice to apply single step closing resistorswhich, for example, in systems of 230 kv. and above were on the order of300 to 500 ohms and were inserted for a period of about 8 milliseconds(one-half cycle). although resistor values and insertion times were notunduly critical. Such single-step closing resistors were effective tolimit surge voltage magnitude to about 210 percent of normal system peakvoltage. It is desirable to provide improved closing resistors whichlimit surge voltages even more so as further reduce the amount of costlyinsulation required, especially as system voltages reach extra highvoltage levels.

In accordance with the present invention, it is possible to limit surgevoltage magnitude to less than 210 percent of normal system peak voltageby employing improved variable closing resistors which have variablerather than fixed value. Such variable resistors have a predeterminedhigh value when initially inserted and decrease in value at apredetermined rate in a predetermined time after insertion. Thepreferred predetermined high value of resistance should be on the orderof four times the normal line surge impedances so as to damp the initialvoltage surge and the resistance value should then be decreased at apredetermined rate to dampen the subsequent voltage surges which arereflected from the system after full round trip travel time until theresistance is finally stepped out.

OBJECTS OF THE INVENTION It is an object of the present invention toprovide improved closing resistors which maintain the lowest possibleswitching surge overvoltages.

Another object is to provide improved closing resistors which havevariable rather than fixed resistance values.

Another object is to provide improved variable closing resistors whichlimit surge voltages to less than 210 percent of normal system voltage.

Another object is to provide improved variable closing resistors whichpermit even further reduction of insulation levels and its associatedequipment as compared to prior art closing resistors.

Another object is to provide improved variable closing resistors whichdrain any charge trapped on the line in accordance with a preferred rateof decay.

Other objects and advantages of the invention will hereinafter appear.

DRAWINGS The accompanying drawings illustrate several preferredembodiments of the invention, but it is to be understood that theembodiments illustrated are susceptible of modifications with respect todetails thereof without departing from the scope of the appended claims.

FIG. 1 is a schematic diagram showing an electrical installationcomprising a multiphase source of electrical power, a transmissionsystem which is adapted to be connected thereto and means comprisingswitches and variable closing resistors in accordance with theinvention;

FIG. 2 is a graph wherein resistance value is plotted against time andwhich illustrates the performance characteristics of three types ofvariable closing resistors in accordance with the present invention;

FIG. 3 is a schematic showing of another type of closing switch andvariable closing resistor usable with the source and system shown inFIG. I; and

FIG. 4 is a schematic showing of a third type of closing switch andvariable closing resistor usable with the source and system shown inFIG. 1.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there isshown an electrical installation comprising a source 10 of electricalpower and an electrical system 20. Source 10 is understood, for example,to be a three-phase high-voltage source which is grounded on one side asat 12 and is provided on its other side with three terminals 14, 16, 18.The terminals l4, 16, I8 of source I0 are adapted to be connected anddisconnected from system 20 which is understood, for example, to be athreephase highvoltage transmission system comprising three lines 22, 24and 26, respectively. It is to be understood. for example, that source10 generates a voltage on the order of 230 kv. or above, and that system20 is of some predetermined length, for example, miles and that thesystem including its associated equipment has inductance designated 28and capacitance designated 30 which can be determined by calculation. InFIG. 1, the inductance in line 22, for example, is hereinafterrepresented by the symbol L and the capacitance is hereinafterrepresented by the symbol C.

Means are provided to connect and disconnect system 20 to source 10 andsuch means take the form of suitable switches 32, 34, and 36 in thelines 22, 24 and 26, respectively, which in practice would be circuitbreakers. disconnect switches, air break switches or the like. Theswitches are shown in FIG. I in open condition.

In accordance with the invention, variable closing resistors 38, 40, 42are shown associated with the lines 22, 24 and 26. respectively. Thevariable closing resistors are adapted to be inserted in series circuitbetween the system and the source for a predetermined length of timeafter the source and system are connected. For purposes of discussion,terminal 14, switch 32 and variable closing resistor 38 associated withline 22 may be considered as typical of corresponding componentsdetailed description of the operation of the components associatedassociated with line 22 will be sufficient to illustrate operation ofthe others.

As FIG. I shows, variable closing resistor 38 comprises a resistiveportion 380 and a contact portion 3812. It is to be understood that asswitch 32 is moved to close, it first makes contact with contact portion38b of closing resistor 38 and sweeps smoothly through the full range ofpredetermined resistance values (from high to low values without steps)built into resistive portion 38a of resistor 38 in a predeterminedamount of time, as FIG. 2 shows in connection with line 39.

In the embodiment shown in FIG. 1, resistance value declines at anexponential rate from a predetermined high-resistance value which isabout four times normal line surge impedance ofline 22, as shown in FIG.2, until it is stepped out of the system. Normal line surge impedance ofline 22 is calculated as VZITC=Z0 and is so designated in the graph inFIG. 2, In practice, a 230 kv. system having a line surge impedance ofZo=3000 would use a variable closing resistor having a high value ofabout 1200.0. which decreases exponentially to a low value of about1600. at which point it is removed from the system and the switch isfully closed. A high value of 4XZo is selected because it will overdampthe first voltage surge of the system and it is decreased exponentiallybecause such rate of decline closely relates to the impedance associatedwith subsequent, progressively smaller voltage surges which arereflected from the system. The time during which resistance valuedeclines exponentially is preferably selected to be about 8milliseconds, the time of a normal half cycle in the line. However, thistime as a practical value may vary depending on the nature of the systemand the practical limits governing switch-closing operations.

FIG. 3 schematically shows a combined switch 32 and a variable closingresistor 44 which may be used instead of the switch 32 and resistor 38of FIG. 1. Resistor 44 comprises resistive portions 44a and contactpoints 4412. Resistor 44 is understood to be of a type where resistancevalues decrease in small incremental steps but at a substantiallyexponential rate, as shown by the line 45 in the graph of FIG. 2.

FIG. 4 schematically shows a combined switch 32 and a variable closingresistor 46 which may be used instead of the type shown in FIGS. 1 and3. Resistor 46 comprises a resistive portion 46a, a contact point 46!),and means 46d whereby the resistor value is automatically varied beforeswitch 32 is moved from contact point 46b to connect directly with line22 at point 46c. In practice, resistor 46 might take the form of awire-wound, carbon pile or liquid resistor. Preferably, resistor 46decreases in value at an exponential rate as shown by line 39 in thegraph of FIG. 2.

While it is preferable that the resistor values be varied at anexponential rate to get maximum surge reduction, voltage surges arereduced well below that obtained from conventional closing resistors, ifresistor value decreases at a linear rate similar to the line 48 shownin the graph of FIG. 2, i.e., within the ohmic values and timesspecified in the graph. Superior performance is also achieved by aresistor having neither a true exponential nor a true linear drop butwhich has a rapid decrease initially and a somewhat slower decrease invalue thereafter, as shown by line 50 in the graph of FIG. 2. 230 kv.and above and having line lengths 230 kv. and above were on the order of300 to 500 It is to be understood, however, that while actualexperiments with resistors having exponentially decaying resistancecharacteristics and resistors having linearly decaying resistancecharacteristics provided the results herein described, resistors havingother nonlinear rates of decay (such as sinetype curves orhyperbolic-type curves) come within the purview of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. An electrical power transmission installation comprising:

a source of electrical power,

a high-voltage electrical system comprising a plurality of spaced-aparttransmission lines and having inductance and capacitance and tending toexhibit voltage surge which initially are several times normal systempeak voltage during the transient period when energized from saidsource,

switch means for connecting said system to said source,

and variable impedance means adapted to be inserted in circuit betweensaid system and said source by said switch means for a time intervalless than one cycle of the fundamental system frequency to reduce saidvoltage surges to less than about 210 percent of the normal peak voltageof the system when said switch means connect said system to said source,

said switch means being adapted to decrease said variable impedancemeans from an initial predetermined value which is greater than aboutfour times normal line surge impedance when first inserted through atleast one value which is smaller than said initial value at apredetermined rate until said impedance means is removed from thecircuit.

2. An installation according to claim 1 wherein said impedance means isdecreased in value by said switch means at a constant rate.

3. An installation according to claim 1 wherein said impedance means isdecreased in value by said switch means at an exponential rate.

4. An installation according to claim 1 wherein said impedance means isdecreased in value by said switch means in incremental steps.

5. An installation according to claim 1 wherein said impedance means isdecreased in value by said switch means first at a rapid rate andsubsequently at a slower rate.

6. An installation according to claim I wherein said impedance means isdecreased in value by said switch means at a rate which is related tothe impedance of successive voltage surges in the system.

7. An installation according to claim 1 wherein said impedance means isdecreased in value by said switch means at a constant rate.

8. An installation according to claim I wherein said impedance means isdecreased in value by said switch means at an exponential rate.

9. An installation according to claim 1 wherein said impedance means isdecreased in value by said switch means in incremental steps.

1. An electrical power transmission installation comprising: a source ofelectrical power, a high-voltage electrical system comprising aplurality of spaced-apart transmission lines and having inductance andcapacitance and tending to exhibit voltage surge which initially areseveral times normal system peak voltage during the transient periodwhen energized from said source, switch means for connecting said systemto said source, and variable impedance means adapted to be inserted incircuit between said system and said source by said switch means for atime interval less than one cycle of the fundamental system frequency toreduce said voltage surges to less than about 210 percent of the normalpeak voltage of the system when said switch means connect said system tosaid source, said switch means being adapted to decrease said variableimpedance means from an initial predetermined value which is greaterthan about four times normal line surge impedance when first insertedthrough at least one value which is smaller than said initial value at apredetermined rate until said impedance means is removed from thecircuit.
 2. An installation according to claim 1 wherein said impedancemeans is decreased in value by said switch means at a constant rate. 3.An installation according to claim 1 wherein said impedance means isdecreased in value by said switch means at an exponential rate.
 4. Aninstallation according to claim 1 wherein said impedance means isdecreased in value by said switch means in incremental steps.
 5. Aninstallation according to claim 1 wherein said impedance means isdecreased in value by said switch means first at a rapid rate andsubsequently at a slower rate.
 6. An installation according to claim 1wherein said impedance means is decreased in value by said switch meansat a rate which is related to the impedance of successive voltage surgesin the system.
 7. An installation according to claim 1 wherein saidimpedance means is decreased in value by said switch means at a constantrate.
 8. An installation according to claim 1 wherein said impedancemeans is decreased in value by said switch means at an exponential rate.9. An installation according to claim 1 wherein said impedance means isdecreased in value by said switch means in incremental steps.